Optical element and method of manufacturing optical element with each of first and second layers having a repetition structure

ABSTRACT

An optical element having a three-dimensional structure which can function in a visible range and can improve adherence at a structural interface of the element, and a method of manufacturing the optical element. The optical element includes a substrate, and at least a first layer and a second layer on the substrate are manufactured such that each of the first layer and the second layer has a repetition structure of spaces and structural parts at a pitch equal to or less than a wavelength of visible light, and at an interface between the first layer and the second layer, overlapped structures are provided in which the repetition structure of the first layer and the repetition structure of the second layer overlap in a stack direction of the layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical element and a method ofmanufacturing the optical element. In particular, the present inventionrelates to an optical element having a three-dimensional hollowstructure such as a polarizing beam splitter, a phase plate, or aband-pass filter which has on a surface thereof a structure with anin-plane period shorter than a wavelength of visible light, and a methodof manufacturing the same.

2. Description of the Related Art

In recent years, optical components having a three-dimensional hollowstructure are actively proposed. To obtain such a three-dimensionalhollow structure will be essential for improving functions of theoptical components in the future. However, a structure of the opticalcomponents is on the order of nanometers, a manufacturing method thereofhas not been established, and there are many practical problems withregard to strength of the element and the like. In order to obtain sucha three-dimensional hollow structure, there is a method using asacrificial layer to manufacture a hollow structure on the order ofmicrometers, that is, so-called MEMS (Micro Electro Mechanical Systems)(see U.S. Pat. No. 4,662,746).

Such a MEMS structure provides a digital mirror device. The digitalmirror device has a hinge for receiving an operating mirror, a yoke forreceiving external forces formed on the hinge, and a mirror fordeflecting external light formed on the yoke. This structure is sized tobe several microns to several hundred microns, the adhesion between anupper layer and a lower layer is adequate, and no practical problem iscaused.

Further, Japanese Patent Application Laid-Open No. 2001-074955 disclosesa structure of a photonic crystal waveguide and a method ofmanufacturing the same. A photonic crystal waveguide is intended toobtain a three-dimensional waveguide by forming structural defects inlayers having a line-and-space structure and stacking them in directionsorthogonal to one another. According to the manufacturing methoddisclosed here, a semiconductor material is used to conduct masstransportation of a semiconductor element at a high temperature to forma junction. At such a material junction, metallic bond or covalent bondis possible, and the upper layer and the lower layer can be stronglyadhered to each other.

A semiconductor material is transparent in an infrared range but opaquein a visible range, and thus, such a semiconductor material cannot beused for an optical element which functions in the visible range.Therefore, it is necessary to use dielectric materials. However, whendielectric materials are heated to a high temperature, it is sometimesdifficult to conduct mass transportation of an element between thedielectric materials to form a junction. In this way, depending on thematerial, it is sometimes difficult to form a junction by heat. Further,when it is attempted to obtain a stacked bottom-up structure using asacrificial layer process, in the case of a nanometer structure of thewavelength equal to or less than that of visible light, the contact areabetween an upper layer and a lower layer becomes extremely small.Therefore, a problem is caused in that the adhesion at the interfacebetween the layers is extremely small and the element is veryvulnerable.

The present invention has been made in view of the above-mentionedproblems. An object of the present invention is to provide an opticalelement having a three-dimensional structure which can function in thevisible range and can improve the adhesion at a structural interface ofthe element, and a method of manufacturing the optical element.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, according to the presentinvention, an optical element having a three-dimensional structureformed as in the following and a method of manufacturing the opticalelement are provided.

According to the first aspect of the present invention, there isprovided an optical element, including: a substrate, and a first layerand a second layer formed on the substrate, wherein each of the firstlayer and the second layer comprises a repetition structure of spacesand structural parts at a pitch equal to or less than a wavelength ofvisible light in a vertical direction to a stack direction of thelayers, and wherein, the repetition structure of the first layer and therepetition structure of the second layer overlap at an interface betweenthe first layer and the second layer, in a stack direction of thelayers.

The optical element according to the first aspect of the presentinvention can further include a plurality of layers between thesubstrate and the first layer, and in the optical element, the firstlayer can be formed of an i-th layer and the second layer can be formedof an (i+1)th layer as counted from the substrate.

In the optical element according to the first aspect of the presentinvention, in the overlapped structures, the repetition structure of thefirst layer and the repetition structure of the second layer can overlapin a range of 3 nm or more and 20 nm or less.

In the optical element according to the first aspect of the presentinvention, each repetition structure of the first layer and the secondlayer can include any one of a line-and-space structure, a structurewith holes, and a structure with dots.

In the optical element according to the first aspect of the presentinvention, the pitch equal to or less than the wavelength of visiblelight in the first layer and the second layer can be 10 nm or more and200 nm or less.

In the optical element according to the first aspect of the presentinvention, the repetition structure of the first layer and therepetition structure of the second layer can be formed of the samematerial.

In the optical element according to the first aspect of the presentinvention, the repetition structure of the first layer and therepetition structure of the second layer can be formed of a dielectricmaterial.

According to the second aspect of the present invention, a method ofmanufacturing an optical element including a substrate, and a firstlayer and a second layer formed on the substrate, the method includingthe steps of: forming the first layer on the substrate, processing inthe first layer a repetition structure comprised of spaces andstructural parts, having a pitch equal to or less than a wavelength ofvisible light, filling the spaces in the repetition structure with amaterial of a sacrificial layer, etching the sacrificial layer to exposean upper portion of the repetition structure from the sacrificial layer,forming the second layer on the repetition structure and the sacrificiallayer, processing in the second layer a repetition structure of spacesand structural parts, having a pitch equal to or less than a wavelengthof visible light, and removing the sacrificial layer.

In the method of manufacturing an optical element according to thesecond aspect of the present invention, in the step of exposing theupper portion of the repetition structure from the sacrificial layer,the side surfaces of the repetition structure can be exposed in a rangeof 3 nm or more and 20 nm or less from an upper surface of therepetition structure.

According to the present invention, the adhesion between an upper layerand a lower layer on a substrate can be improved, and poor adhesion canbe prevented. Further, in a manufacturing process or the like, when, forexample, external forces act, problems peculiar to a microstructure suchas pattern collapse can be prevented.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C are schematic views for illustrating aconfiguration of an optical element having a three-dimensional structureaccording to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a substrate and a pattern arrangementaccording to Example 1 of the present invention.

FIGS. 3A and 3B are diagrams illustrating pattern shapes of a firstlayer and a second layer of a three-dimensional structure of an opticalelement according to Example 1 of the present invention.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H and 4I are schematiccross-sectional views for illustrating a manufacturing process of theoptical element having the three-dimensional structure according toExample 1 of the present invention.

FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, 5H and 5I are schematiccross-sectional views for illustrating a manufacturing process of anoptical element having a three-dimensional structure according toExample 3 of the present invention.

FIG. 6 is a graph for comparing phase difference characteristics of aphase plate according to Example 3 of the present invention with phasedifference characteristics of a phase plate made of quartz.

FIG. 7 is a graph illustrating average transmittance of the phase plateaccording to Example 3 of the present invention and that of a phaseplate according to Comparative Example 2.

FIGS. 8A, 8B, 8C and 8D are schematic views for illustrating a processfor sticking a substrate to a prism according Example 4 of the presentinvention.

FIG. 9 is a graph illustrating spectral transmittance of S-polarizedlight and P-polarized light of a polarizing beam splitter according toExample 4 of the present invention.

FIG. 10 is a diagram illustrating another example of an optical elementaccording to Example 4 of the present invention.

FIG. 11 is a view illustrating an exemplary pattern of a line-and-spacestructure.

DESCRIPTION OF THE EMBODIMENTS

The embodiment of the present invention will be described below. FIGS.1A to 1C are schematic views for illustrating a configuration of anoptical element having a three-dimensional structure according to thisembodiment. FIG. 1A is a perspective view, FIG. 1B is a cross-sectionalview seen from a direction indicated by the arrow i, and FIG. 1C is across-sectional view seen from a direction indicated by the arrow ii. Inthis embodiment, as shown in FIGS. 1A to 1C, an optical element includesa stacked structure in which a repetition structure of spaces (air gaps)201 and structural parts 202 of the first layer (a first structure) 2and a repetition structure of the second layer (a second structure) 3 ofspaces (air gaps) 301 and structural parts 303 are stacked on asubstrate 1. In this case, a plurality of layers may be provided betweenthe substrate 1 and the first structure 2, and the structure 2 may beprovided as an i-th layer counting from the substrate. Further, aplurality of layers may be provided as an upper layer on the secondstructure 3.

Here, when the structure 2 is formed of the i-th layer counting from thesubstrate, each of the i-th layer and an (i+1)th layer is processed of arepetition structure of spaces and structural parts at a pitch equal toor less than the wavelength of visible light in a vertical direction toa stack direction of the layers. At an interface between those layers,the pattern structures overlap in a stack direction thereof. Morespecifically, at the interface between the i-th layer (first structure2) and the (i+1)th layer (second structure 3), a repetition structurehaving a pitch equal to or less than the wavelength of visible light ofthe i-th layer and a repetition structure having a pitch equal to orless than the wavelength of visible light of the (i+1)th layer overlapin a stack direction thereof. In other words, the repetition structuresengage into each other. The state that the repetition structures engageinto each other is hereinafter referred to as “overlapped state”, andthe structures which engage into each other are hereinafter referred toas “overlapped structures”. Further, a portion 4 where the structuresengage into each other is hereinafter referred to as “overlappedportion”.

According to this embodiment, the overlapped structures can make largerthe contact area between the i-th layer and the (i+1)th layer, whichmakes it possible to improve the adhesion between the layers. Further,the existence of such an overlapped portion can prevent minutestructural defects such as pattern collapse. Here, at the interfacebetween the above-mentioned i-th layer (the first structure 2) and the(i+1)th layer (the second structure 3), the overlapped portion 4 wherethe overlapped state is the largest is preferably in the range of 3 nmor more and 20 nm or less. In the case where the largest overlap portionis less than 3 nm, when an in-plane distribution in manufacturing is notuniform, there is a possibility that poor adhesion is partially caused.Further, in the case where the largest overlap portion is less than 3nm, the contact area between the upper and lower layers is small, andadequate adhesion strength cannot be obtained. On the other hand, whenthe largest overlapped portion is more than 20 nm, planarity of the(i+1)th layer is inadequate, and optically adverse effects such as lightscattering are generated. Further, because the refractive index of theoverlapped portion is a value between that of the i-th layer and that ofthe (i+1)th layer, an optically thick overlapped portion cannot obtaindesired characteristics.

Further, in this embodiment, the above-mentioned repetition structureshaving a pitch equal to or less than the wavelength of visible light ofthe i-th layer and the (i+1)th layer can be formed as any one of aline-and-space structure, a structure with holes, and a structure withdots. In the case of a line-and-space structure, a pattern isanisotropic with regard to a polarized component of light, and thus, itis an effective structure in obtaining a configuration such as apolarizing beam splitter or a low-pass filter. Further, lines of theline-and-space structure may be divided at a pitch equal to or less thanthe wavelength of visible light (see FIG. 11). By the division, peelingoff of the pattern due to a layer stress can be prevented. Further, inthe case of a structure with dots where the pattern is a column-shape ora structure with holes where the pattern is a hole-shape, the pattern isisotropic with regard to a polarized component of light, and thus, it iseffective as a component of an antireflection film of a multilayerinterference film or of a band-pass filter.

Here, by making the pitch of the repetition structures equal to or lessthan the wavelength of visible light, a diffraction phenomenon at awavelength used for the optical element can be prevented, and thus,optically stable characteristics can be obtained. Here, it is desirablethat the pitch of the repetition structures is 10 nm or more and 200 nmor less. In particular, when the pitch is 150 nm or less, diffractionlight is not generated in an optical element using an incident angle of45° which functions in a visible light region where the wavelength is400 nm or more, and thus, the optical element functions effectively. Ifthe pitch is less than 10 nm, it is difficult to maintain the structuresas such. Further, effectiveness of a layer formed of spaces (hereinafterreferred to as “air gaps”) and structural parts particularly resides inthat a desired refractive index can be obtained. Generally, eachmaterial has its own refractive index, and it is difficult to obtain anarbitrary refractive index. However, when a layer is formed of air gapsand structural parts, an arbitrary refractive index can be obtained bycontrolling the ratios of the air gaps and the structural parts.

More specifically, the refractive index can be theoretically controlledto be in a range from the refractive index of the material of thestructures to a value larger than the refractive index of air, i.e.,larger than 1. In particular, the refractive index of magnesium fluoridewhich can be stably used as a layer having a low refractive index is1.38. However, when silicon oxide (refractive index: 1.46) is used asthe structures and the ratio of the air gaps is 90 percent, and astructure with holes is formed, the refractive index is 1.146, and thus,a layer having a very low refractive index which cannot be attained witha dielectric can be obtained. Further, when a line-and-space structureis formed also with a ratio of the air gaps of 90 percent, therefractive index becomes still lower and anisotropy of the refractiveindex appears. The refractive index with regard to an electric field(vibration component of light in parallel with the direction of thelines) is 1.055 while the refractive index with regard to a magneticfield (vibration component of light perpendicular to the direction ofthe lines) is 1.028.

Further, in this embodiment, the structural material of the i-th layerand of the (i+1)th layer can be the same. When the structural materialof the i-th layer and the (i+1)th layer are the same in this way,because the influence of an refractive index in between the layersdescribed above can be neglected, it is easier to obtain desiredcharacteristics. Further, in this embodiment, a dielectric can be usedfor the structural material. It is desirable that the material of anoptical element which functions in the visible range does not absorblight in the visible range. Many dielectrics are transparent in thevisible range, and effective as the structural material of the opticalelement according to the present invention. In particular, siliconoxide, titanium oxide, tantalum pentoxide, zirconium oxide, and the likeare effective materials because they are easily etched in an etchingprocess.

Next, a method of manufacturing an optical element having athree-dimensional structure according to this embodiment is described.First, in a process of forming the repetition structure of the i-thlayer, having a pitch equal to or less than the wavelength of visiblelight, after patterning is carried out using photolithography, etchingis carried out. In the photolithography, exposure may be carried out byany method which is not limited particularly as long as it can obtain adesired pitch, for example, a stepper, an Electron Beam drawingequipment, an X-ray exposing apparatus, or an interference exposingapparatus. Further, since the etched structures are minute, it isdesirable to use dry etching. The dry etching can be carried out by anymethod which is not limited particularly as long as it can obtain adesired pitch, and the dry etching may be, for example, RIE (ReactiveIon Etching), ICP (Inductively Coupled Plasma), or NLD (Neutral LoopDischarge). Wet etching may also be carried out as long as a desiredpitch can be obtained. When a dielectric is etched with a resist usingas the mask, selectivity is sometimes a problem. In this case, it isdesirable that the mask for etching the structures is a multi-layeredmask such that the selectivity can be obtained.

Next, a process of filling the spaces with a material of a sacrificiallayer can be carried out by applying a commonly used coating technique.For example, spin coating, spray coating, or slit coating may be used.Further, the material of the sacrificial layer may be any material whichcan be ashed with oxygen, such as a photoresist material, a BARC (BottomAnti—Reflection Coating) material, an acrylic resin, or a polystyreneresin. Further, in order to improve the planarity of the surface of thesacrificial layer, it is desirable that the thickness from an uppersurface of the structures is large. On the other hand, in order to makeshorter the time necessary for a planarizing process, it is desirablethat the thickness is small. Therefore, it is desirable that thethickness of the sacrificial layer from the upper surface of thestructures is 50 nm or more and 200 nm or less. In order to improve theplanarity of the sacrificial layer, it is effective to apply thesacrificial layer for multiple times.

Next, it is desirable that in a process of etching (etching back) thewhole surface of the sacrificial layer to expose an upper portion of thei-th repetition structures from the sacrificial layer, commonly used dryetching is employed. For example, a parallel plate type RIE apparatusmay be used. A side surface of the repetition structures in a range of 3nm or more to 20 nm or less from the upper surface of the repetitionstructures is exposed from the sacrificial layer. The amount of theetching can be controlled by the etching time. Here, oxygen is used asan etching gas. When the oxygen is pure oxygen, because the etching rateis high, controllability of the etching amount may become bad. By mixingCF₄ or CHF₃Cl gas with oxygen, the etching rate can be made lower toimprove the controllability of the etching amount.

Next, a process of forming the (i+1)th layer on the i-th layer may becarried out by using a commonly used film forming technique. Forexample, vapor deposition, sputtering, or CVD may be used. It is to benoted that, in order not to allow the sacrificial layer to be deformedor deteriorated in quality, the process temperature has to becontrolled. Next, a process of forming the repetition structures of the(i+1)th layer, having a pitch equal to or less than the wavelength ofvisible light, is carried out similarly to the case of the i-th layer.Because the etching is carried out until the sacrificial layer isexposed, the upper portion of the i-th layer pattern is etched to adepth equal to the range of the height of the overlapped portion (seethe reference numeral 4 of FIG. 1C). Alternatively, the upper portion ofthe i-th layer pattern may be etched in a range of the height of theoverlap portion or more as far as the optical characteristics permit.This forms a difference in level between the i-th pattern and the(i+1)th pattern, and thus, it is easier to remove the sacrificial layer.Finally, a process of removing the sacrificial layer which fills thespaces in the i-th layer may be carried out by dry etching. For example,commonly used etching with an RIE apparatus using pure oxygen may beused. Alternatively, an ashing apparatus solely for the resist may beused. This process can also be controlled by the time. By the methoddescribed above, an optical element having a three-dimensional structurecan be obtained.

As described above, according to this embodiment, the adhesion betweenan upper layer and a lower layer on the substrate can be improved, and apoor adhesion can be prevented. Further, in a manufacturing process orthe like, for example, when external forces act, problems peculiar to amicrostructure, such as pattern collapse, can be prevented. Stillfurther, when a large area with a diameter of 6 inches, 8 inches, or thelike is processed at the same time, because the whole surface can beeffectively used, the number of elements which can be obtained from thearea becomes larger, which enables efficient production. Further, whilea thick overlapped portion affects optical characteristics, by makingthe overlapped portion in a range of 20 nm or less, desired opticalcharacteristics can be easily obtained.

Examples of the present invention are now described in the following.

EXAMPLE 1

First, a substrate and a pattern arrangement of a three-dimensionalstructure of an optical element of this example are described withreference to FIG. 2 and FIGS. 3A and 3B. Nine patterns 6 each of whichhas a size of 25 mm×25 mm were patterned on a substrate 5 which is a6-inch quartz wafer. Here, as illustrated in FIG. 3A, the patterns inthe first layer were at a pitch of 0.26 μm, the diameter of the holepatterns 7 was 0.13 μm, and the hole patterns 7 were arranged atvertices of regular triangles. As illustrated in FIG. 3B, the patternsin the second layer were at a pitch of 0.26 μm, the diameter of the holepatterns 8 was 0.2 μm, and the hole patterns 8 were arranged at verticesof regular triangles. Although, in this example, the holes were formedusing the same mask and the hole diameters were controlled by the amountof exposure, the mask may be changed.

Next, a manufacturing process of the optical element having thethree-dimensional structure is described. FIGS. 4A to 4I illustrate themanufacturing process of the optical element according to this example.First, patterning of the first layer is described. After a 6-inch quartzwafer substrate 9 was cleaned and dried, a silicon oxide film was formedto have a thickness of 100 nm by sputtering to form a silicon oxidelayer 10. As a result, the layer for forming the structure of the firstlayer was obtained (FIG. 4A). Then, a photolithography step forpatterning the silicon oxide layer 10 was carried out. Here, as aphotoresist for the patterning, Clariant AX6850P was used. The resistwas applied by spin coating, and the coating was carried out such thatthe thickness of the film was 300 nm. After the coating, prebaking wascarried out at 110° C. for two minutes. Then, exposure was carried outwith a stepper FPA-5000-ES4b manufactured by Canon Inc. As for theexposure pattern, hole patterns at a pitch of 0.26 μm were used in the25 mm×25 mm area. The diameter of the holes was 0.13 μm and the holepatterns were arranged at vertices of regular triangles. The amount ofexposure in this case was 32 mJ/cm². The exposure was carried out atnine points in the 6-inch substrate. After the exposure, PEB (PostExposure Bake) was carried out at 120° C. for two minutes.

Then, the substrate having the structure of the first layer was soakedin a developer containing 2.38% of TMAH (tetramethyl ammonium hydroxide)for one minute, and the developer was rinsed out by pure water shower toobtain hole patterns 11 of the resist (FIG. 4B). Then, the silicon oxidelayer was etched. The etching was carried out by a parallel plate typeRIE apparatus with CHF₃ used as an etching gas under a pressure of 2.7Pa with an RF power of 100 W (0.3 W/cm²) for 4.3 minutes. Further, inorder to remove residual resist, ashing was carried out with oxygen gasused as an etching gas under a pressure of 2.7 Pa with an RF power of100 W for one minute. In this way, silicon oxide hole patterns 12 wereobtained with a hole depth of 100 nm (FIG. 4C).

Next, filling and planarizing steps are described. As a fillingmaterial, AZ Exp. KrF-17C8 manufactured by Clariant Company was used.The filling was carried out by spin coating. After the spin coating at2500 rpm for thirty seconds, prebaking was carried out at 180° C. forone minute. This was repeated two times to complete the filling (FIG.4D). As a result, a planarized interface 13 of the filled layer wasobtained at a position of 50 nm above an upper surface 14 of the siliconoxide hole patterns. Planarization was carried out with an apparatussimilar to that used for the etching. Ashing was carried out with amixture of oxygen gas (17 vol %) and CHF₃ (83 vol %) used as an etchinggas under a pressure of 3 Pa with an RF power of 20 W (0.06 W/cm²) for5.5 minutes. Measurement by an AFM clarified that a planarized substrate(FIG. 4E) was obtained with an exposed amount of the structure ofsilicon oxide (which is the height of the side surfaces 141 of thesilicon oxide patterns exposed from an upper surface of the sacrificiallayer to the upper surface 14 of the patterns, as indicated by referencesymbol A in FIG. 4E) being 3 nm.

Next, patterning of the second layer is described. In patterning thesecond layer, a silicon oxide film was formed as the second layer forthe substrate to have a thickness of 10 nm by sputtering. As a result, acontinuous and uniform silicon oxide layer 15 with sufficient planaritywas able to be obtained (FIG. 4F). Then, a photolithography step forpatterning the second silicon oxide layer was carried out. This wascarried out similarly to the case of the first silicon oxide layerexcept that the amount of exposure was 50 mJ/cm². The patterns after thedevelopment were at a pitch of 0.26 μm and the diameter of the holes was0.2 μm. Then, the second silicon oxide layer was etched similarly to thecase of the first silicon oxide layer except that the etching time was0.5 minutes. Further, in order to remove residual resist, ashing wascarried out with oxygen gas used as an etching gas under a pressure of2.7 Pa with an RF power of 100 W for one minute. In this way, siliconoxide hole patterns 16 were obtained with a hole depth of 10 nm (FIG.4G). FIG. 4G shows a schematic cross-sectional view taken along the linea-a′ of FIG. 4I.

Next, ashing of the sacrificial layer is described. The ashing of thesacrificial layer was carried out using an RIE apparatus utilizingoxygen gas under a pressure of 3 Pa with a power of 100 W for threeminutes. The sacrificial layer filling the holes in the first layer wasremoved to obtain hollow structure 17 (FIG. 4H). The sacrificial layerwas vaporized by plasma and removed from very minute gaps 18 (FIG. 4I).The material of the sacrificial layer is required to have the followingmaterial characteristics. That is, the material needs to be solid atordinary temperatures, and when dissolved in an organic solvent, thematerial needs to be a material which can be made into a thin film byspin coating or spray coating. Basically any material which can bedecomposed into a gas having a high vapor pressure by using oxygenplasma can be used. In this way, the two-layered three-dimensionalstructure of silicon oxide was obtained. With regard to all the ninepatterns in the 6-inch surface, the 25 mm×25 mm areas where the patternswere formed appeared to be uniform, and, even after nitrogen blow of 0.5MPa was carried out, the appearance did not change, and it was asatisfactory structure. Further, observation of a section taken alongthe center of the pattern with an FE-SEM confirmed that, at theinterface between the first layer and the second layer, the two layerswere strongly adhered to each other.

EXAMPLE 2

In Example 2, the planarizing step was carried out for six minutes.After the planarization, the upper portion of the silicon oxidestructure was exposed by 20 nm. The second silicon oxide layer had athickness of 70 nm, and the second silicon oxide layer was etched for3.5 minutes. In the same conditions as in Example 1 except for theabove, a two-layered three-dimensional structure of silicon oxide wasobtained. After the process of forming the second silicon oxide layer, asection was observed. The result was that, although the surface had aconcave-convex height of about 5 nm, a continuous film was formed andpost processes was able to be carried out similarly. With regard to allthe nine patterns in the 6-inch surface, the 25 mm×25 mm areas where thepatterns were formed appeared to be uniform, and, even after nitrogenblow of 0.5 MPa was carried out, the appearance did not change, and itwas a satisfactory structure. Further, observation of a section takenalong the center of the pattern with an FE-SEM confirmed that, at theinterface between the first layer and the second layer, the two layerswere strongly adhered to each other.

EXAMPLE 3

In Example 3, similarly to the case of Example 1, an optical glasssubstrate which is a 6-inch wafer was used to carry out exposure at ninepoints. FIGS. 5A to 5I are views for illustrating a manufacturingprocess of a three-dimensional structure of an optical element accordingto this example. First, an S-TIH53 optical glass substrate 19manufactured by Ohara Inc. was cleaned, and then, vapor deposition of atantalum pentoxide layer 20 was carried out such that the film thicknesswas 960 nm (FIG. 5A). Then, as a mask material for etching tantalumpentoxide, a WSi layer 21 was formed by sputtering. Then, as a maskmaterial for etching the WSi layer 21, a silicon oxide layer 22 wasformed. The multilayer mask layer is effective when, in an etching stepdescribed below, appropriate selectivity cannot be secured between aphotoresist and a layer to be etched. The thickness of the WSi layer andthe thickness of the silicon oxide layer were 200 nm and 120 nm,respectively.

Then, as a mask for etching the silicon oxide layer, a photoresistpattern was formed. An exposure step of the photoresist pattern wascarried out using an interference exposure method. Here, because WSi isused in the multilayer mask, light returned by the rear surface inexposure becomes strong. Therefore, the returned light and incidentlight interfere with each other, and thus, there is generated a problemin that the sectional shape of the photoresist after the exposure andthe development does not become a rectangle. Therefore, a BARC layer 23was provided to absorb the returned light by the rear surface so thatthe sectional shape of the photoresist after the exposure and thedevelopment becomes a rectangle. The photoresist used here was UV-170manufactured by Shipley Company. The BARC used here was AZ Exp. KrF-17C8manufactured by Clariant Company. The respective materials were spincoated on the substrate having the multilayer mask materials formedthereon. The BARC was prebaked at 180° C. for one minute, and the filmthickness here was 115 nm. The photoresist was prebaked at 100° C. fortwo minutes, and the film thickness here was 140 nm.

Then, the substrate was exposed to light by using two-beam interferenceexposure method. A light source had a wavelength of 266 nm which isfourth harmonic of Nd-YAG (Neodymium : yttrium-aluminum-garnet) wasused. The angle of incidence on the substrate was 56°. Laser beams wereexpanded by 100 times by a beam expander. The amount of exposure was 30mJ/cm². In the case of three-beam interference exposure, it is possibleto form the hole pattern in Example 1. After the exposure, PEB (PostExposure Bake) was carried out at 120° C. for 1.5 minutes. Then, afterthe substrate was soaked in a solution containing 2.38% of TMAH (TetraMethyl Ammonium hydroxide) for 30 seconds, the solution was rinsed outby pure water shower to obtain a photoresist pattern 24 (FIG. 5B). Here,a pattern effective area of 35 mm×35 mm was able to be secured.

Then, the BARC, silicon oxide, WSi, and tantalum pentoxide layers wereetched. The etching was carried out using an ICP apparatus. The BARC andsilicon oxide layers were etched using an etching gas similar to that inExample 1 under similar etching conditions. The WSi layer was etchedwith a mixed gas of SF₆ and chlorine at a ratio of 1:2 under a pressureof 2.7 Pa with an RF power of 1.5 W/cm² for 40 seconds. After that, thetantalum pentoxide layer was etched with SF₆ as an etching gas and witha bias of 20 W applied on the substrate side under a pressure of 6 Pawith a power of 1.2 W/cm² for 50 minutes to obtain a line-and-spacestructure 25 of tantalum pentoxide (FIG. 5C). Here, the lines, thespaces, and the pitch of the line-and-space structure were 130 nm, 30nm, and 160 nm, respectively. Then, filling and planarizing steps werecarried out. The filling step was carried out similarly to the case ofExample 1. As a result, a filled substrate on which a planarizedinterface 26 of the filling layer was obtained at a position of 50 nmabove an upper surface of the tantalum pentoxide pattern was obtained(FIG. 5D). Then, the planarizing process was carried out in a waysimilar to that in Example 1 for 5.6 minutes. As a result, a planarizedsubstrate was obtained, in which side surfaces 271 of the tantalumpentoxide structure were exposed by 5 nm in height from an upper surfaceof the sacrificial layer to the upper surface 27 of the tantalumpentoxide structure. Then, the second layer and the third layer wereformed (FIG. 5E). After vacuum vapor deposition was carried out toevaporate a tantalum pentoxide layer 28 to have a thickness of 24 nm, asilicon oxide layer 29 was evaporated to have a thickness of 10 nm (FIG.5F).

After that, a multilayer mask was formed, and a photoresist pattern wasformed in a way similar to the case of the first layer by using aninterference exposure method. The amount of exposure here was 25 mJ/cm².After that, in a way similar to the case of the first layer, themultilayer mask, the silicon oxide layer, and the tantalum pentoxidelayer were etched. The etching of the silicon oxide layer and thetantalum pentoxide layer was carried out at the same time underconditions similar to those of the etching of tantalum pentoxide of thefirst layer. The etching time was 1.5 minutes. FIG. 5G shows a schematiccross-sectional view at that time. The schematic cross-sectional view isa cross-sectional view seen from the direction indicated by the arrow iof FIG. 5F. This illustrates that the pattern in the first layer and thepattern in the second layer are orthogonal to each other. Here, thelines, the spaces, and the pitch of the line-and-space structure 30 were96 nm, 64 nm, and 160 nm, respectively.

Then, similarly to the case of Example 1, the sacrificial layer wasashed to obtain a three-dimensional optical element having a hollowstructure 31 (FIG. 5H). This schematic cross-sectional view of FIG. 5His a view seen from the direction indicated by the arrow ii of FIG. 5G.With regard to all the nine patterns in the 6-inch surface, the 35 mm×35mm areas where the patterns were formed appeared to be uniform, and,even after nitrogen blow of 0.5 MPa was carried out, the appearance didnot change, and it was a satisfactory structure. Further, observation ofa section taken along the center of the pattern with an FE-SEM (FieldEmission-Scanning Electron Microscope) confirmed that, at the interfacebetween the first layer and the second layer, the two layers werestrongly adhered to each other.

The optical element according to this example functions as a phaseplate. FIG. 6 is a graph for comparing phase difference characteristicsof a phase plate according to this example with phase differencecharacteristics of a commonly used phase plate made of quartz. Solidblack squares in FIG. 6 illustrate phase difference characteristics of aconventional quartz phase plate while solid black circles illustratephase difference characteristics of the phase plate having the structureillustrated in FIG. 5I. The result shows that a change in the phasedifference of the phase plate according to this example in the visiblerange is smaller than that of the phase plate made of quartz, and thus,the phase plate according to this example is excellent in opticalcharacteristics. FIG. 7 shows the result of measurement of averagetransmittance in the visible range. FIG. 7 shows that the averagetransmittance in the visible range is near 100%, and thus, anantireflective effect can be obtained at the same time. Theantireflective effect is thought to be efficiently exhibited when thestructure of silicon oxide provided in the last layer functions as alayer having a low refractive index.

EXAMPLE 4

In Example 4, an optical glass substrate similar to the above-mentionedone in Example 3 was used and a titanium oxide layer was formed bysputtering to have a thickness of 360 nm. Then, similarly to the case ofExample 3, a multilayer mask was formed thereon by sputtering. Themultilayer mask was composed of chromium in a thickness of 50 nm andsilicon oxide in a thickness of 80 nm. Then, similarly to the case ofExample 3, after a BARC and a photoresist were applied, patterning andetching were carried out to obtain a structure of a first layer,provided that the angle of incidence on the substrate of interferenceexposure was 72°, the amount of exposure was 35 mJ/cm², and chromium wasetched with a mixed gas of chlorine and oxygen at a ratio of 1:3 used asan etching gas and with a bias of 120 W applied on the substrate sideunder a pressure of 6 Pa with an RF power of 50 W for one minute andforty seconds. The titanium oxide layer was etched for 25 minutes. Inthis way, the structure of the first layer was obtained. Here, thelines, the spaces, and the pitch of the line-and-space structure of thefirst layer were 30 nm, 110 nm, and 140 nm, respectively.

Then, filling and planarizing steps were carried out similarly to thecase of Example 3. In the filling step, a similar material was used,and, after spin coating was carried out at 1000 rpm for 30 seconds,prebaking was carried out at 180° C. for one minute. This was repeatedtwo times to complete the filling. As a result, a planarized interfaceof the filling layer was obtained at a position of 200 nm above an uppersurface of the titanium oxide pattern. The planarization was carried outsimilarly to the case of Example 3, and ashing was carried out for 22minutes. The result of measurement by an AFM of the exposed amount oftitanium oxide at the respective centers of the nine patterns on the6-inch optical glass substrate illustrated in FIG. 2 was 20 nm withregard to patterns 6-1, 6-3, 6-7, and 6-9, 15 nm with regard to patterns6-2, 6-4, 6-6, and 6-8, and 10 nm with regard to a pattern 6-5.

Then, a second layer was formed. Similarly to the case of the firstlayer, the second layer was formed with using a sputtering method byforming a film of titanium oxide with a thickness of 70 nm and, forminga multilayer mask composed of a chromium layer with a thickness of 50 nmand a silicon oxide layer with a thickness of 80 nm. Then, a resistpattern was formed. The amount of exposure was 18 mJ/cm². The structureof the second layer was formed under conditions similar to those of thefirst layer except that the etching time of titanium oxide was fiveminutes. The lines, the spaces, and the pitch of the line-and-spacestructure of the second layer were 120 nm, 20 nm, and 140 nm,respectively.

Then, similarly to the case of Example 3, a sacrificial layer was ashedto form a hollow structure of the first layer. Further, the filling andplanarizing processes similarly to the case of the first layer werecarried out. A titanium oxide layer having a thickness of 360 nm wasformed on the substrate by sputtering. The third titanium oxide layerwas patterned similarly to the case of the first layer. The lines, thespaces, and the pitch of the line-and-space structure were, similarly tothe case of the first layer, 30 nm, 110 nm, and 140 nm, respectively.Finally, similarly to the case of the first layer, a sacrificial layerwas ashed to form a hollow structure of the second layer. The process offorming the third layer could be carried out to form a continuous filmthereon, even when the third film was directly formed without using theprocess of forming a sacrificial layer, because the spaces of the secondlayer were small.

With regard to all the nine patterns in the 6-inch surface, the 35 mm×35mm areas where the patterns were formed appeared to be uniform, and,even after nitrogen blow of 0.5 MPa was carried out, the appearance didnot change, and it was a satisfactory structure. Further, observation ofa section taken along the center of the pattern with an FE-SEM confirmedthat, at the interface between the first layer and the second layer, thetwo layers were strongly adhered to each other, and, at the interfacebetween the second layer and the third layer, the two layers werestrongly adhered to each other. Then, the obtained substrate of athree-layered structure was scribed to carve out a rectangle of 28.3×20mm, and nine substrates having the three-dimensional structure wereformed. Each of the nine substrates was stuck to an S-TIH53 substrate.

FIGS. 8A to 8D are schematic views for illustrating a process forbonding the substrates. First, an S-TIH53 substrate 32 was cleaned (FIG.8A). Then, an adhesive layer was spin coated and temporary curing wascarried out. As the adhesive, PLENACT KR-55 which is a titanate couplingagent and manufactured by Ajinomoto-Fine-Techno Co., Inc. diluted by afactor of 60 with isopropyl alcohol was used. The spin coating wascarried out at 5000 rpm for 30 seconds and the temporary curing wascarried out at 180° C. for two minutes to obtain the substrate with anadhesive layer 33 (FIG. 8B). Then, the above-mentioned scribed substrate34 having the three-dimensional structure was overlaid on the adhesivelayer such that the portion having the structure portions is in contactwith the adhesive layer. Then, the overlaid substrate was left on a hotplate under a load of 2 kg for five minutes at 200° C. (FIG. 8C). Afterthe substrate was cooled, the substrate was adhered to 45° prisms 35made of S-TIH53 so as to be sandwiched therebetween, and a prism wasobtained (FIG. 8D).

The optical element according to this example functions as a polarizingbeam splitter. FIG. 9 illustrates spectral transmittance of S-polarizedlight and P-polarized light with regard to incident angles of 45°±10° ofthe prism according to this example. As illustrated in FIG. 9, there wasno problem except that the transmittance of S-polarized light wasincreased when the incident angle was 35° and the transmittance ofP-polarized light was decreased when the incident angle was 55°. Inparticular, when the incident angle was 45°±5°, the characteristicshardly changed, and it can be seen that the optical element according tothis example was excellent in optical characteristics (Because thecharacteristics at the incident angles of 40° and 50° were the same asthose at the incident angle of 45°, the plotting of the characteristicsat the incident angles of 40° and 50° were omitted in FIG. 9). Althoughthe three-dimensional structure is formed to the ends of the substratein FIG. 8, there may be a space between an end of the substrate and astructure as illustrated in FIG. 10. Because such a configuration cansecure a space at an element opening, thermal damage due to an abrupttemperature change can be alleviated and gas discharge from a bondedpart can be suppressed. This enables a more stable projected image interms of environmental circumstances to be obtained.

COMPARATIVE EXAMPLE 1

In Comparative Example 1, in a process of planarizing a filled layer ofa first layer, ashing was carried out for 20 minutes. Here, the resultof measurement by an AFM of the exposed amount of titanium oxide at therespective centers of nine patterns on the 6-inch optical glasssubstrate illustrated in FIG. 2 was 1 nm with regard to patterns 6-1,6-3, 6-7, and 6-9; −4 nm with regard to patterns 6-2, 6-4, 6-6, and 6-8;and −9 nm with regard to a pattern 6-5. Here, the minus values indicatethat titanium oxide was not exposed and a sacrificial layer remained onthe surface thereof. After that, processes similar to those of Example 4were carried out. In the patterns 6-2, 6-4, 6-6, 6-8, and 6-5, lightscattering was caused after the sacrificial layer was ashed. As a resultof measuring a section of a defect portion with an FE-SEM, the titaniumoxide layer of a second layer had a defect on the entire surface in aminute area. Further, because there was no inconvenience in terms of theappearance, the patterns 6-1, 6-3, 6-7, and 6-9 were sandwiched betweenprisms. During standing at 200° C. in a step of post-baking an adhesive,peeling off from an interface was observed.

COMPARATIVE EXAMPLE 2

In Comparative Example 2, planarization for 6.1 minutes was carried out.In this case, the amount of exposure of tantalum pentoxide was 25 nm.Under the conditions similar to those of Example 3 except for the above,a phase plate was obtained. As a result, the phase plate was apparentlyin a fogged state. Further, when spectral transmittance was measured, itwas confirmed that, as illustrated by solid black triangles in FIG. 7,the transmittance was decreased.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2006-052013, filed Feb. 28, 2006, and Japanese Patent Application No.2007-032708, filed Feb. 13, 2007, which are hereby incorporated byreference herein in their entirety.

1. An optical element comprising: a substrate; and a first layer formedon the substrate; and a second layer formed on the first layer, whereinthe first layer comprises a repetition structure of spaces and linearstructural parts at a pitch equal to or less than a wavelength ofvisible light, wherein the second layer comprises a repetition structureof spaces and linear structural parts at a pitch equal to or less than awavelength of visible light, wherein the linear structural parts of thesecond layer are formed so as to cross over the spaces of the firstlayer, wherein the linear structural parts of the first layer areelongated in a direction so as to contact multiple linear structuralparts of the second layer, and wherein upper portions of the structuralparts of the first layer and lower portions of the structural parts ofthe second layer engage into each other in a range of 3 nm or more and20 nm or less.
 2. An optical element according to claim 1, furthercomprising a plurality of layers between the substrate and the firstlayer, wherein the first layer is formed of an i-th layer and the secondlayer is formed of an (i+1)th layer counting from the substrate.
 3. Anoptical element according to claim 1, wherein the pitch equal to or lessthan the wavelength of visible light in the first layer and the secondlayer is 10 nm or more and 200 nm or less.
 4. An optical elementaccording to claim 1, wherein the repetition structure of the firstlayer and the repetition structure of the second layer are formed of thesame material.
 5. An optical element according to claim 1, wherein therepetition structure of the first layer and the repetition structure ofthe second layer are formed of a dielectric material.